In centerless grinding, workpieces in the form of right cylinder are ground by the through-feed method, in which workpieces placed between a grinding and a regulating wheels on a workrest are fed automatically in the axial direction, whereby the axes of the grinding and the controlling wheels are held at a predetermined distance during the grinding of workpieces. In case of the centerless grinding of workpieces in the form of, for example, a motorshaft, an axially stepped cylinder, or the workpieces, in the form of bolt and valve stem with the head at the end of the shaft, loaded workpieces are ground by the in-feed method, in which the depth of cut of a grinding wheel on the workpiece is controlled by the in-feed of the regulating wheel to the grinding wheel. With the tangential feed method, a workpiece between a grinding and a regulating wheels is fed in the tangential direction of the circumference of the grinding wheel, so as to place the workpiece in the grinding position and to give the depth of cut of the grinding wheel on the workpiece.
A workpiece ground the tangential feed method has the same shape as a workpiece ground by the in-feed method. As compared with the in-feed method, the tangential feed method has the following advantages, the tangential feed movements provide for the setting of the depth of cut on the workpiece by means of a fixed distance between the axes of the grinding and regulating wheels, as well as providing for the loading and unloading of the workpiece at the workrest. For the reason as stated above, the setting of the depth of cut is thus easily made with high accuracy. Moreover, the machine can be simply constructed for fully automatic operation, and the cycle time for the centerless grinding of each workpiece is remarkably reduced. Nevertheless, in case of the centerless grinding machine employing the tangential feed method, which is different from by the other centerless grinding machines which employ the through-feed or the in-feed method, it is difficult to solve such problem as the lowering in the rigidity of the workrest, and also to realize the mechanism of the workrest movement which occurs during the grinding operation.
The inventor has therefore developed the tangential feed method for centerless grinding, and has studied the feed mechanisms, whereby the workrest is placed between the grinding and the regulating wheels, and is rotated or reciprocated with unequal velocity when the workpieces are loaded, ground and unloaded.
As a result, in case of the rotating workrest in the form of a thin cylinder with a number of holding positions of the workpiece on the cylindrical circumference, the grinding operation is continued completely and automatically with an considerable saving of time. However, the rigidity of thin cylindrical workrest, which is fixed to the side of the rotating disk, is insufficient and as the cylindrical workrest is made for the given workpiece with a fixed dimension and shape, the use of this machine is limited to specific workpieces. Also, the roundness of the ground workpieces does not sufficiently approach a true circle, because the workpieces are ground by passing them through the line which joins the centers of the grinding and regulating wheel axes.
The centerless grinding employing tangential feed method, in which the workrest is reciprocatively moved, is advantageous in that it is easy to select the geometrically optimum grinding conditions, so as to correspond to the centerless grinding machine when grinding workpieces of different dimensions and shapes then in the case of a rotating workrest. As for the saving of grinding time, is is difficult to achieve the unequal velocity of the reciprocating workrest with a suitable driving mechanism in accordance with the loading, grinding and unload of the workpiece.
In another case, at the Nomoco Co. in West Germany, the centerless grinding machine employing the tangential feed method with a reciprocating workrest was developed several years after the experiments of the inventor. In this case, in order to solve the technical difficulty of achieving reasonable unequal velocities of the workrest, and especially to avoid the effect of the lowering in rigidity of the workrest using the tangential feed movement of the workrest for the loading, grinding and unloading of the workpiece, the grinding and regulating wheels are arranged on the side wall of the upright column of the machine whereas the grinding and regulating wheels are usually set firmly on the bed of the machine. Also, the workrest is arranged on the horizontal guide way, which is perpendicular to the side wall of the upright column, and the direction of the movements of the workrest for loading and grinding is applied against the cutting direction of the grinding wheel. During the centerless grinding, in conventional centerless grinding machines, the workrest supports the workpiece in a wedge-like space, which is formed by the cylindrical outer surfaces of the grinding and regulating wheels, and prevents the tendency of the workpiece to pull away the wedge-like shape in opposition to the cutting force.
In this case, however, the workrest, in contrast to the conventional machine, pushes the workpiece towards the cutting force in the wedge-like space, and so, by exceeding the depth of cut or at the beginning of the grinding or workpieces which are not completely round, the workrest disturbs or prevents the smooth rotation of the workpieces, so that the contact pressure of the workrest on the workpiece must be kept elastic in the direction of the feed movement.
With the constructional feature of this centerless grinding machine, the workrest does not require a high degree of rigidity, during grinding, and the driving movements of the workrest, according to the loading, grinding and unloading of the workpiece can easily be controlled by the unequal velocity and the risk of a break-down in the grinding wheel can also be avoided, even though the rigidity of the workrest may not be sufficient in the directon of the feed movement.
In this centerless grinding machine, as mentioned above, during centerless grinding, the supporting conditions of the workpiece and the construction of the machine, where the grinding and regulating wheels are arranged vertically on the wall of the column, have the following defects, so that it is difficult to obtain a centerless grinding machine with maximum stability, high rigidity and efficient operative characteristics, that is:
1. As mentioned above, during centerless grinding, by the fact that the workrest must maintain elastic contact with the workpiece and there is a lack of rigidity in the direction of the feed movement, the setting of the heavy grinding condition is impossible for the grinding operation with high efficiency. Also, becaue of the excessive depth of cut, the rotational movement of the workpiece lacks smoothness, and because of the harmony of the periodical displacement of the workrest, the formation of the workpiece in a shape not completely round is accelerated and the workpiece is formed to an unround shape called "Greichdick" having three or other odd number convex portions in the circumference.
2. The construction, in which the grinding and regulating wheel heads are arranged on the side wall of the upright column, is inferior to the conventional machine construction, in which the grinding and regulating wheel heads are arranged on the bed, from the viewpoint of the stational and dynamic rigidity and stability of the machine. Therefore, because of the increased depth of cut or extended grinding width, the dimensional accuracy and accuracy of shape decrease as the grinding and regulating wheel spindles are displaced at the increased grinding resistance, and also the realization of a high degree of productivity is very difficult.
3. As the aforesaid constructional principles for centerless grinding machine are disregarded the method of driving and the precise settings of the grinding and regulating wheels and the ways of operating the different centerless machine parts are limited. Therefore, the production of a high performance centerless grinding machine with high operational and stable characteristics is very difficult.